Browsing by Author "Andonova, S."
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Item Open Access Chemical deactivation by phosphorous under lean hydrothermal conditions over Cu/BEA NH3-SCR catalysts(Elsevier, 2014-04-05) Andonova, S.; Vovk, E.; Sjöblom, J.; Ozensoy, E.; Olsson, L.To obtain a better understanding of the deactivation of SCR catalysts that may be encountered due to the presence of P-containing impurities in diesel exhausts, the effects induced by P over Cu/BEA NH3- SCR catalysts were studied as functions of the temperature of poisoning and P concentration in the feed. Cu/BEA catalysts with different Cu loadings (4 and 1.3 wt% Cu) were exposed to P by controlled evaporation of H3PO4 in the presence of 8% O2 and 5% H2O at 573 and 773K. The reaction studies were performed by NH3-storage/TPD, NH3/NO oxidation and standard NH3-SCR. In addition, a combination of several characterisation techniques (ICP–AES, BET surface area, pore size distribution, H2-TPR and XPS) was applied to provide useful information regarding the mechanism of P deactivation. Pore condensation of H3PO4 in combination with pore blocking was observed. However, the measured overall deactivation was found to occur mostly by chemical deactivation reducing the number of the active Cu species and hence deteriorating the redox properties of the Cu/BEA catalysts. The process of P accumulation on the surface preferentially occurs on the “over exchanged” Cu active sites with the formation of phosphate species. This is likely the reason for the more severe deactivation of the 4% Cu/BEA compared to 1.3% Cu/BEA. Further, the higher NOx reduction performance at 773K of the P-poisoned Cu/BEA catalysts was found to originate from the lower selectivity towards NH3 oxidation, which occurs predominately on the “over-exchanged” sites.Item Open Access Effects induced by interaction of the Pt/CeO x /ZrO x /γ-Al 2 O 3 ternary mixed oxide DeNO x catalyst with hydrogen(Elsevier, 2020) Andonova, S.; Ok, Zehra Aybegüm; Özensoy, Emrah; Hadjiivanov, K.Effects of H2/D2 adsorption on the surface chemistry of Pt/CeOx-ZrOx/γ-Al2O3 DeNOx catalyst were investigated. In-situ FTIR spectroscopy and NOx-TPD techniques were utilized to monitor changes in the surface chemistry of studied materials. Adsorption studies of CO and O2 revealed that the Pt/Ce-Zr/Al sample, initially reduced with H2 at 723 K, is characterized by the presence of oxygen vacancies in close vicinity of Ce3+ centres and metallic Pt sites. Adsorption of O2 occurred through the formation of superoxide (O2 −)ads species and oxidation of Ce3+ to Ce4+ ions. The ability of the catalyst to activate molecular O2 originates from its relatively high population of oxygen vacancies located on/near the surface. Interaction of Pt/Ce-Zr/Al system with H2 or D2 takes place through heterolytic dissociation at ambient temperature. D2 adsorption leads to the reduction of Ce4+ to Ce3+ ions and formation of adsorbed molecular heavy water and gradual D/H exchange with the existing surface hydroxyl groups. Generated D2O interacts with isolated hydroxyls/deuteroxyls through H-bonding and this provokes the formation of H-bonded OeH/OeD groups. These later species are relatively stable and gradually vanish with increasing temperatures above 523 K, leaving behind only isolated hydroxyls. Surfaces enriched with H-bonded hydroxyls are characterized with an enhanced NOx storage ability revealing their significant role in low-temperature NOx adsorption mechanism.Item Open Access Pt/CeOx/ZrOx/γ-Al2O3 ternary mixed oxide DeNOx catalyst: surface chemistry and NOx interactions(American Chemical Society, 2018) Andonova, S.; Ok, Zehra Aybegüm; Drenchev, N.; Özensoy, Emrah; Hadjiivanov, K.Surface chemistry and the nature of the adsorbed NOx species on a Pt/CeO2-ZrO2/Al2O3 catalyst were investigated by IR spectroscopy, X-ray diffraction, H2-temperature programmed reduction, and NOx-temperature programmed desorption. Parallel studies were also carried out with benchmark samples such as CeO2/Al2O3, ZrO2/Al2O3, CeO2-ZrO2/Al2O3 and Pt-supported versions of these materials. All samples were studied in their reduced and nonreduced forms. The use of CO as a probe molecule revealed that during the synthesis of the mixed-metal oxide systems, deposited zirconia preferentially interacted with the alumina hydroxyls, while deposited ceria was preferentially located at the Lewis acid sites. Despite the limited extent of Zr4+ ions incorporated into the CeO2 lattice, the reduction of ceria was promoted and occurred at lower temperatures in the presence of zirconia. When deposited on ZrO2/Al2O3, platinum formed relatively big particles and existed in metallic state even in the nonreduced samples. The presence of ceria hindered platinum reduction during calcination and yielded a high platinum dispersion. Subsequent reduction with H2 led to the production of metallic Pt particles. Consequently, NO adsorption on nonreduced Pt-containing materials was negligible but was enhanced on the reduced samples because of Pt0-promoted NO disproportionation. The nature of the nitrogen-oxo species produced after NO and O2 coadsorption on different samples was similar. Despite the high thermal stability of the NOx adsorbed species on the ceria and zirconia adsorption sites, the NOx reduction in the presence of H2 was much more facile over Pt/CeO2-ZrO2/Al2O3. Thus, the main differences in the NOx reduction functionalities of the investigated materials could be related to the ability of the catalysts to activate hydrogen at relatively lower temperatures.Item Open Access Structure and properties of KNi–hexacyanoferrate Prussian Blue Analogues for efficient CO2 capture: Host–guest interaction chemistry and dynamics of CO2 adsorption(Elsevier, 2021-06-04) Andonova, S.; Akbari, S. S.; Karadaş, Ferdi; Spassova, I.; Paneva, D.; Hadjiivanov, K.Potassium Nickel hexacyanoferrate Prussian Blue Analogues (K-NiFe-PBAs) offer an excellent platform for efficient CO2 capture due to their porous nature and accessible channels. Herein, the effect of Ni:K atomic ratio on the structure and the CO2 storage capacity was studied by employing K-NiFe-PBAs with Ni:K ratio of ca. 2.5 and 12. The porosity and the isosteric heat of CO2 adsorption can be modulated and optimized by varying the Ni:K atomic ratio in the PB framework and thus, covering the thermodynamic criterion for easy CO2capture and release with acceptable energy costs. The synthesized K-NiFe-PBAs containing only trace amounts of K+ ions (with Ni:K = 12) shows an adsorption capacity (∼3.0 mmol g–1 CO2 at 273 K and 100 kPa) comparable to other well established CO2 adsorbents. In situ FTIR spectroscopy was further employed to elucidate the host–guest interaction chemistry and the dynamics of K-NiFe-PBAs within CO2 and H2O. The analysis enabled, to the best of our knowledge, is the first FTIR spectroscopic observation of the high sensitivity of the material to structural distortions induced by small changes under water vapor pressure. It was found that H2O hardly affects CO2 adsorption and the materials are perspective for CO2 capture in the presence of water.